1. Field of the Invention
This invention pertains to a composition especially adapted for treatment of mammalian skin, in particular, human skin. The composition is characterized by a low pH, and suitable for topical administration to the skin.
2. Background of the Prior Art
Mammalian skin, and human skin in particular, has a normal pH generally in the range of 4-6. A variety of agents that are commonly encountered and used for health and/or beauty care, including soaps and detergents, tend to raise skin pH above its general value. The skin acts as a general buffer zone or protectant against outside agents, and a change in the pH may hamper or limit the ability of the skin to provide its protective functions. Recovery of pH in the skin, after exposure to soaps and the like, is particularly retarded in certain classes that might be considered "at risk".
One common form of skin-treatment compositions can be characterized as oil-in-water emulsions, in which an oleaginous phase, predominantly hydrocarbon, generally present in a minor amount, is prepared in an emulsion with water or an aqueous phase. To maintain the stability of such emulsions, a surfactant is commonly employed. Nonionic and anionic surfactants are familiar to those of skill in the art. In general cationic surfactants, such as cetyltrimethylammonium bromide, although sometimes used as such as cetyltrimethylammonium bromide, although sometimes used as active agents in anti-bacterial preparations, are not conventionally used in skin treatments, due to their irritation potential. Indeed, cetyltrimethylammonium bromide (CTAB) is considered a strong irritant to the skin according to the Federal Health and Safety Association (FHSA). A rare exception to the strongly irritating nature of this class of surfactants is distearyldimmonium chloride, which has a markedly lower irritant characteristic, and is currently used in skin care products such as EVERSOFT.TM. (Andrew Jergens Company), CUREL.TM. and SOFTSENSE.TM. (Bausch & Lomb) and AVEENO.TM. (S. C. Johnson). Nonetheless, wider scale use of cationic surfactants in skin care emulsions would be desirable, for a variety of reasons. Cationic surfactants carry a positive charge. This is advantageous in light of the negatively charged characteristic of skin. Moreover, nonionic surfactants do not thicken emulsions, and require specific preservatives for use. Anionic surfactants carry a negative charge. Anionic surfactants have been demonstrated to cause damage to the stratum corneum barrier. Indeed, sodium lauryl sulfate, an anionic surfactant, is often used as a positive control in irritation tests.
Due to the high irritant character of cationic surfactants, their use in the prior art in compositions for topical application has been limited, principally, to compositions for the treatment of hair. Hair, like skin, is substantially negative, and thus substantively attractive to the cationic surfactant. Among the prior art directed to such compositions include U.S. Pat. No. 5,019,376, Uick, described as being particularly useful as hair conditioning compositions, with a pH of 2-5, and U.S. Pat. No. 4,252,695, Homma et al, with a pH of 5, and again devoted to hair conditioning. Other hair conditioning art includes U.S. Pat. No. 4,711,776, Suzuki et al, U.S. Pat. No. 5,143,518, Medrange et al, and U.S. Pat. No. 5,248,445, Rizvi et al. Hair colorants are described in U.S. Pat. No. 4,168,144, Curry et al. Medrange and Curry are directed to combinations of cationic surfactants and fatty acids which combinations exhibit basic pH values and are unsuitable for the treatment and preservation of the natural pH of human skin. None of the references identified discloses a low pH skin conditioning treatment. U.S. Pat. No. 4,710,314, Medrange et al, is also directed to a composition using cationic silicone polymers, but not the cationic surfactants of the claimed invention. This case is again directed to hair treatment compositions.
A different type of skin-conditioning composition, which is not an emulsion, is addressed in International Publication WO93/07856, Deckner et al, which is directed to a polyacrylamide gel, specifically indicated to be nonionic. The reference specifically states that surfactants are not be used, page 10, lines 28-31. The reference does establish a variety of reasons for preparing low pH skin-treatment compositions, which include, in addition to augmenting or enhancing the buffering action of skin in response to alkaline materials, using the low pH formulations as better vehicles for skin-active agents which are applied topically, such as keratolytic agents, anti-acne agents and the like. There are a variety of pharmaceuticals which exhibit low (acidic) pH values, and thus are better suited to a low pH carrier.
Fatty acids (saturated and unsaturated) constitute a class of compounds commonly used in many skin care products as opacifiers, thickeners and emulsion stabilizers. The general structure for saturated fatty acids is C.sub.n H.sub.2n CO.sub.2, while unsaturated fatty acids contain at least one double bond, C.sub.n H.sub.2n-x CO.sub.2. In general saturated fatty acids with more than 10 carbon atoms are solid at room temperature, and may be unsuitable for use in liquid emulsion skin care compositions. Unsaturated fatty acids are generally liquid at room temperature. Stearic acid is a widely used saturated fatty acid, generally used in the form of sodium stearate. This compound finds wide application as a soap emulsifier. In general, fatty acids appear as mixtures of other closely related acids. Commercially available stearic acid contains a mixture of C.sub.18 and C.sub.16 acids. Similarly, palmitic acid is a mixture of C.sub.16 and C.sub.14 acids.
Accordingly, it remains a goal of those of skill in the art to provide a low pH skin care composition that can be used to address or neutralize the effects of alkaline materials such as soaps and detergents which cause the skin pH to rise above normal skin pH of 4-6.
A further object of those of skill in the art is to find a method for using cationic surfactants in skin care emulsions without causing skin irritation. An additional object of the art is the provision of a skin care composition with low skin irritation, low pH and employing fatty acids as an agent therein.